1. Field of the Invention
The present invention relates to the field of electrical power generation, more particularly to the field of hybrid electrical power generation wherein at least a portion of the power generated is derived from sources other than fossil fuels.
2. Background of the Art
Generation of electricity for purposes of transmitting the electricity over the power grid has traditionally been performed by the burning of fossil fuels for heat energy, transferring that heat energy to water to boil the water into superheated steam, and then expanding the superheated steam through a steam turbine and extracting useful energy from the steam through a generator coupled to the steam turbine. Fossil fuels such as coal, oil, natural gas, lignite and the like have been combusted to provide heat to heat the water into steam. Alternatively, nuclear or solar based heat has been used to heat water for steam based electricity generation. An additional fossil fuel based generation scheme includes the combusting of natural gas in a gas turbine engine, which is coupled to an electrical generator, to create electricity.
It is also known in the art to combine heat or energy sources, such as solar, with non-solar generation sources, such as fossil fuel based sources to generate electricity. Such combined source generation facilities are commonly referred to as hybrid generation facilities.
The worldwide majority of commercial level electric power produced from Solar and provided to the electric grid from a centralized plant is provided by the SEGS (Solar Electric Generating Systems) plants built by LUZ International between 1985 and 1991. The SEGS power stations, which are located in Daggett, Boron and Hinkley, Calif., are all hybrid Solar-Thermal plants. Each of these plants employ a common solar energy collection strategy, which is based upon collecting the solar radiation through parabolic mirrors, creating a “Line” of intensely focused and collected sunlight, which is directed along and into a pipe which is transparent to at least some of the wavelengths of the focused and collected sunlight. This light energy is absorbed or collected in a heat transfer fluid (HTF) which is flowed through the pipe, the fluid being heated to a temperature on the order of 400° C. at the pipe exit. This heat transfer fluid is then pumped into a water vaporizer (Heat Exchanger), where the heat in the HTF is exchanged into the water, i.e., the water comes in cold and leaves hot and the HTF comes in hot and leaves cooler. This results in boiling of the water and the generation of steam, which is then directed through piping to a steam turbine-generator set, the generator of which is coupled to a transformer to supply its power to the local commercial electric grid.
To enable the SEGS plants to be reliable power generating facilities, i.e., a generating facility which can produce electricity at its rated capacity in the presence of full sunlight, during periods of fluctuating solar insolation, and when no sunlight is present, SEGS plants are permitted, by government regulation (which allows the electricity generated to still be considered “solar” for regulatory purposes), operate from heat provided from sunlight and alternatively from heat provided from fossil fuel so long as two additional criteria are met:                The SEGS plant must be able to produce its full rated capacity of electric power from solar radiation only, and if this condition is met, then        The SEGS plant is allowed to produce (and sell as solar) electrical energy so long as no more than 25% of the thermal input energy producing the electrical output energy is provided from fossil fuels.        Additionally, under FERC, a SEGS plant is allowed to use only 25% of its total energy produced by burning Fossil Fuel (In most cases that fuel is Natural Gas), and FERC does not permit the usage of that gas to power a Gas Turbine as part of the Solar field power block        
Typically, to provide an electrical generating facility having both solar and fossil fuel energy sources which also meet the above standards, a gas-fired heater/boiler is placed between the solar field and the heat exchanger. By utilizing the heater/boiler, a steady and constant production of power can be guaranteed, especially during periods of “Peak” demand, despite fluctuations in solar radiation.
Such a Hybrid electrical generating facility is shown in FIG. 1, wherein the solar field 1 consists of a number of SCA's 1A (Solar Collector Assemblies). These SCA's are basically a light reflecting structure with high tensional stiffness, having capability to move and accurately track the sun's position throughout the day, which tracking is accomplished by a controller and drive system which are known in the art. The SCA's support an array of parabolically shaped mirrors, which as a group produce a “focal line,” i.e., they together form a focused line of light at their focal point, which is intended to be positioned within the cylindrical boundary or envelope of a pipe in the SCA through which the heat transfer fluid is flowed. The pipe or Heat Collection Element (HCE) is placed collinear with, and in a position to surround, the focused line of sunlight, thus enabling the collecting of the solar insolation energy by heating the HTF passing through the pipe to about 400° C. as it exits the pipe. The Heat Transfer Fluid (HTF), which is typically a mineral or silicon based oil having a relatively high heat capacity and high boiling point, is pumped through the collection side of the system by the Pump 6, which first pumps the HTF from the collection pipe through a Buffer Tank 2, (where hot HTF may be temporarily stored to be used to supplement for minor fluctuations in solar insolation intensity). Exiting from Tank 2, the HTF enters into the Solar Superheater 14, (where the steam coming from the heat exchanger 3 on the water cycle side of the system is further heated to a higher temperature before entering to the second stage of the steam turbine). Exiting from the Solar Superheater 14, the HTF flows to the Heat Exchanger 3. In this heat exchanger the thermal energy carried by the HTF, is transferred to the water which turns the water into steam. The HTF exiting the Heat Exchanger passes through an Expansion Vessel 5, and thence is passed back to the Solar Field, thus creating a closed loop flow of the HTF fluid in the generation system. The water passing through the Heat Exchanger 3, is converted into steam, which flows through the Solar Superheater 14, and then into the second stage of the Steam Turbine 7 where the steam is expanded and drives the steam turbine to drive the Generator 8 to output electric power which is fed to the Grid 9.
The (water saturated) steam being exhausted from the Steam Turbine 7 flows into the Recuperate (Steam Separator) 13, and then flows into the Water Cooled Condenser 12. In the Condenser the steam condenses back into water, which is pumped back through the Separator 13 to the non-operating Gas Heater 4. When solar insolation is at its peak, the condensed water passes through the Gas Heater 4, back to the Heat Exchanger 3. However, when solar insolation is not sufficient, and Natural Gas can be burned (according to the regulations of the Federal Energy Regulatory Commission, or FERC), the Gas Heater 4 is used to create superheated steam which is directed to the first stage of the steam turbine, and thus working in a closed loop through the Steam Turbine 7. In order to cool the steam in the Condenser 12, water is pumped by Pump 11 from the Condenser through a Cooling Tower 10 where it is cooled by ambient air.
The most valuable time for delivering power (In the present SEGS power plants, the buyer is Southern California Edison (SCE)), is during “On Peak” hours which are defined as weekdays except holidays from June 1st to September 30th, from Noon until 18:00. The number of “On Peak” hours varies from 504 to 516 hours annually.
A typical SEGS power plant delivers approximately 2,000 full capacity equivalent hours of electricity over a period of approximately 2,800 hours during the year. Approximately 1,500 of those full capacity equivalent hours come from solar insolation and approximately 500 full capacity equivalent hours comes from burning Natural Gas in the HTF heater, i.e., as the SEGS is capable of providing full rated capacity solely based on solar energy, it is allowed to generate, and sell as solar produced energy, up to 25% of its output from fossil fuel energy, in this case, natural gas.
The 1,500 solar sourced hours of electrical production are delivered based on the availability of incident light from the sun reaching the solar field. The 500 hours of natural gas produced electricity has typically been generated on a 1st priority basis to meet any additional heat requirements to augment the solar field in order to meet Contract On-Peak Capacity requirements, and on a 2nd priority basis the natural gas would be burned Mid-Peak in the summer hours which are the most valuable delivery non On-Peak hours. As a third priority, natural gas would be used to supply heat energy during Mid-Peak winter hours.
The current state of the art uses the solar field under typical On-Peak (peak utility company demand), Mid-Peak (mid peak utility company demand), and Off-Peak (minimum utility company demand) operations to run the entire power block, and therefore the solar field in sized so that from several hours before peak solar insolation to several hours after peak solar insolation the solar fields provide sufficient heat to power the entire power block.